(Adapted from the applicant's abstract): PPH is a disease of high morbidity and mortality occurring predominately in young adult women. The etiology of this illness remains unknown, but increased production of thromboxane A(2) [TxA(2)] and decreased synthesis of prostacyclin [prostaglandin I] provide clues to the pathogenesis. Over the past decade, intravenous epoprostenol, the synthetic analogue of prostacyclin, has emerged as the most effective treatment of PPH. However, tolerance to the effects of epoprostenol occurs in the majority of patients necessitating progressive dose escalation to maintain efficacy. Furthermore, only 70% of patients benefit from treatment. Preliminary data derived from clinical studies of patients with PPH demonstrate that epoprostenol increases circulating levels of angiotensin II (AII), a potent vasoconstrictor and smooth muscle mitogen, which can stimulate production of both plasminogen activator inhibitor 1 (PAI- 1), a procoagulant protein, and vascular endothelial growth factor (VEGF), permeability and angiogenic growth factor. This proposal will explore two hypotheses: 1) activation of the renin- angiotensin system (RAS) during chronic administration of epoprostenol is the cause of increasing dose requirements; 2) direct and indirect effects of RAS activation and persistent TxA(2) production limit the clinical efficacy of epoprostenol. To evaluate these hypotheses, the applicant will: a) delineate the relationship between epoprostenol-induced RAS activation and compare biochemical changes with hemodynamic data obtained during right heart catheterization; b) delineate clinical data obtained from measurement of distance walked in six minutes, and structural changes obtained by wedge angiography of pulmonary circulation; and c) determine, in a collaborative study with other medical centers, whether concomitant treatment with and angiotensin converting enzyme inhibitor will improve the clinical efficacy of epoprostenol and prevent the need for chronic dose escalation. These studies will advance our knowledge of the mechanism of action of epoprostenol and pulmonary hypertension.